When you have ported the project to TypeScript, you can refactor the program more easily. You should remove patterns that do not fit well with TypeScript. For instance, magic string values should be replaced by enums. When you have a project that uses a framework, you can also do some framework related refactoring. For a React project, you might want to upgrade from the old class creation with React.createClass to the new class syntax.

A proper editor can help during refactoring. VS Code can rename an identifier in the whole project or find all references of an identifier. It can also format your code if it's messy or jump to the definition of an identifier. You can access these options with a right-click on the identifier in the code.

Which steps you must do for refactoring depends on your project. You should look for parts of the code that are not typed or incorrectly typed, because of a bad structure or some dynamic behavior.